Add audit log to registrant verifications

Closes #1425

app/models/domain.rb

@@ -57,6 +57,7 @@ class Domain < ApplicationRecord
 
   has_many :legal_documents, as: :documentable
   accepts_nested_attributes_for :legal_documents, reject_if: proc { |attrs| attrs[:body].blank? }
+  has_many :registrant_verifications, dependent: :destroy
 
   after_initialize do
     self.pending_json = {} if pending_json.blank?

app/models/registrant_verification.rb

@@ -1,6 +1,8 @@
 # Used in Registrant portal to collect registrant verifications
 # Registrant postgres user can access this table directly.
 class RegistrantVerification < ApplicationRecord
+  has_paper_trail
+
   # actions
   CONFIRMED = 'confirmed'
   REJECTED  = 'rejected'